1. Field of the Invention
Nitric acid is an important industrial commodity; most of which goes into agricultural fertilizers, largely in the form of ammonium nitrate.
The alchemists of the later middle ages were acquainted with the chemical properties of nitric acid, which they called Aqua Fortis or strong water or sometimes Aqua Valens or Powerful Water. Early nitric acid was made by heating strong sulphuric acid with saltpeter.
The production of nitric acid by distilling sodium nitrage with sulphuric acid is an essentially simple operation with the formula of: (at 900.degree. C.). EQU 2naNO.sub.3 + H.sub.2 SO.sub.4 = HNO.sub.3 + Na.sub.2 SO.sub.4
a lessor efficient method is: (at 200.degree. C.). EQU naNO.sub.3 + H.sub.2 SO.sub.4 = NaHSO.sub.4 + HNO.sub.3
2. Description of Prior Art
Commercial nitric acid was made in a battery of retorts connected to condensers with absorption towers for recovery of vapors and a variety of accessory equipment. The retort is a large cast iron vessel, cast in two or more sections, in a brickwork foundation, and an arrangement for heating. The usual charge is 4000 pounds of nitrate of soda. Life of the retort is 550 to 600 runs.
Connection to a condensor is made by earthenware pipes. The condensor consists of one or more stands, each containing inclined glass tubes connected to a baffled stoneware header, arranged so the hot vapors made several passes. The condenser tubes are water cooled.
The usual cycle of a retort is 14 hrs. The strength of the distillate produces a 92.5% to 97.0% acid.
Nitric acid is now produced exclusively by oxidation of ammonia. Ammonia is produced by the hydrolysis of calcium cyanide (Ca(CN).sub.2 + 3H.sub.2 = CaCO.sub.3 + 2NH.sub.3); LeChatelier's principal (3H.sub.2 + N.sub.2 + Pres = 2NH.sub.3); synthesis of natural gases from organic decomposition, as well as industrial processes such as coke production, etc.
Ammonia is oxidized to nitric oxide which when mixed with water produce nitric acid.
This reaction takes place as a series of bimolecular reactions, its rate of operation controlled by the rate ammonia can be physically brought into contact with a catalyst. Ammonia conversions as high as 97.5%, with a four-layer cylindrical gauze catalyst of pure platinum, at atmospheric pressure, with 8.3% ammonia in air, preheated to give a gauze temperature of 930.degree. C. at a rate expressed as 100 pounds of NH.sub.3 per troy ounce of catalyst per (24 hr.) day.
The oxidation of NH.sub.3 to NO requires a further phase; that is, the conversion of NO to NO.sub.2 which will react with water to form nitric acid HNO.sub.3. The essential steps are in sequence -- (1) cooling the reaction products to a point which causes most of the water produced in the reaction to condense; (2) allow the gases, with a mixture of some additional air to give an excess of oxygen over that required to convert the NO to HNO.sub.3 ; then (3) bringing the gases into contact in successive countercurrent stages with water while allowing extended time of residence to convert to NO.sub.2 then NO formed upon its reaction with water.
A high pressure system also produces nitric acid. Principal advantage of the pressure system is smaller plants, and greater percentage nitric acid.